As its last measure the present parliament will approve its silliest. It will "ban" a recently discovered party drug called mephedrone. MPs will declare next week that, while they may have been venal, spendthrift and corrupt, at least their final act will have protected thousands of young innocents from the devil. They will have well and truly banned something. They will feel much better, and go off whistling into the night.

They are the only ones who will feel better. The reason for their contentment is that they have responded to a headline of a tear-stained family pleading for a drug to be banned after the sad death of a daughter after taking it. If nowadays the public wants something banned – other than alcohol and cigarettes, which MPs enjoy – then it will be. Perhaps the outcome will be no different, indeed a sign is that MPs will declare they are merely "sending a signal". But something will have been done about a headline, the only stimulus to action known to the Home Office.

The shenanigans now enveloping drug classification and scheduling under the 1971 Misuse of Drugs Act have become absurd. The Advisory Council on the Misuse of Drugs, set up under the act to advise ministers on harm classification, has valiantly tried to honour its mission. But since its advice is purely advisory and can be overruled, its work is polluted. Its members have acquired the aura, in some cases unfairly, of Soviet scientists ordered to doctor their disciplines to toe the party line.

That is why the council's David Nutt was sacked last year as its chairman, after questioning aspects of government drugs policy. He accepted that ministers were entitled to do whatever they liked, but they could hardly object if he expressed his own opinion, in this case in an academic lecture. Six members of Nutt's committee resigned and a seventh, Polly Taylor, went this week. She was ordered to sign an Orwellian statement that could only have been penned in home secretary Alan Johnson's bunker, that she "should not act to undermine mutual trust" between herself and Johnson. Paranoia has driven British government to this pass.

The irony is that the rump council was about to recommend just what the home secretary wanted, that mephedrone be made illegal within the terms of the 1971 act and placed in class B with other amphetamines. It would attract a prison sentence for possession and a maximum of 14 years for dealing.

There are now two drugs regimes operating in Britain, each divorced from the other. One is "policy", as presented before parliament and in the pages of the press, fought out among government advisers, spin doctors and MPs, rising and falling with public hysteria. This policy believes that it has now eradicated a menace from the face of Britain, saving tens of thousands of young people from death and degradation at the hands of their worst natures and an army of pushers and dealers.

The other regime is the real one – that of drug users, their desperate parents and friends, together with teachers, social workers, police, club owners, dealers and their suppliers, all in chaotic and lawless relationships with each other. To them the government's decision will have one consequence. It will reportedly take mephedrone from roughly £2 a go to £40.

The fallacy, indeed fantasy, of drug legislation is that prohibiting supply prohibits demand. Not only is the foolishness of this thesis staring MPs in the face, from four decades of statutory failure, it applies to the hundreds of new "offences" parliament declares every year that are virtually unenforceable, usually in response to some headline.

Yet each measure has an effect. It drops the marker dye of criminality into the economic blood stream. It distorts the pattern of demand and supply and, in its ineffectiveness, subverts respect for authority. Nobody I know who is conversant with the drugs scene, even those in favour of a "clampdown", regards the present law as anything other than an out-of-date nuisance. Britain has no workable drug laws, merely legislation that randomly fills jails with those unlucky enough to get caught, and ruins thousands of families more completely than the impact of the drugs themselves.

If the evidence of other amphetamines is any guide, the classification of mephedrone will impose a high-risk premium on the supplier, and thus a cost on the user. Where the drug produces an addiction that is left untreated, users will turn to crime to pay the premium, vastly increasing the profit to the supplier, and thus increasing supply. There is no mystery about this. Thanks to the 1971 act Britain has wide experience of the impact of criminalisation on the drugs economy. As Nutt himself said in response to Johnson's ban: the criminals will be "rubbing their hands".

That drugs, legal and illegal, are dangerous should not be an issue. Enough is known about mind-affecting substances for sensible people not to use them, and certainly not too much of them. That some drugs, such as marijuana, can be used safely by some people, in the same way as alcohol, cannot detract from the risk attached to them. But that applies to many things people use, especially young people. Over 7,000 each year die of alcohol "overdosing" and tens of thousands from nicotine poisoning. Amphetamines kill something like 100 a year, but barely a dozen deaths have been attributed to mephedrone – far less than paracetamol.

There is anecdotal evidence that users of more dangerous drugs, such as heroin and cocaine, switched to mephedrone not because it was legal but because it was cheap. Some researchers were studying its burgeoning use in the hope of seeing whether such "legal high" chemicals might be quality controlled and supplied under licence with health warnings, reducing dependence on more serious drugs. It was thus possible that mephedrone might have offered a guide to a route out of the hell that feeble home secretaries have inflicted on millions of people. No one can claim any amphetamine is safe but, if it is legal, it is easier to control.

Alan Johnson has closed off that possibility. In driving the supply of a popular drug underground, he has reinforced the failed policy of prohibition, a policy that made interwar Chicago an economy ruled by crime. Like cocaine and heroin, mephedrone will now suffer adulteration. While fewer users may be able to afford it, those who like it will turn to crime to pay for it, and to the health service when they need to recover from overdose or adulteration.

Two years ago the UK Drug Policy Commission concluded: "There is little evidence … that drug policy influences either the number of drug users or the share of users who are dependent." A year ago Transform, a drug policy thinktank, used the government's own figures to calculate that drugs prohibition is costing Britain £14bn in health, crime, imprisonment and family breakdown, a cost that legalisation and control could save. It is a cost that Johnson has just increased.

I bet this is one cost no politician will propose saving at next month's election.